Biotic Control

What are the aims of biotic control?

To manage the presence and populations of species to maximise agricultural productivity

What are pests and how might they affect agricultural productivity?

Pests are organisms e.g. insects like caterpillars and locusts, birds, rodents and larger mammals like elephants which reduce agricultural productivity or the quality of the product. They may be predators that eat the crop or livestock, compete for resources like water or nutrients, be pathogens that cause disease which can potentially impact breeding cycles/success, carry pathogens, and reduce marketability such as by spoiling the appearance of fruit or by weed seeds mixing with the cereal harvest.

Name and describe three general methods that might be used to control pests

• Pest control using pesticides and toxic chemicals to manage populations

• Biological control, such as the use of the Encarsia Formosa wasp to control whitefly - this is particularly effective as they disrupt the whitefly reproductive cycle, reducing populations long-term

• Setting up barriers such as crop nets or fences to restrict pest movements - this is a technique that is used for larger mammals like elephants

What are the main groups of species that are agricultural pests?

Weeds, Insects, Fungi, Bacteria, Molluscs, Nematode worms, Vertebrates

What problems may be caused by weeds and are there any examples for this?

Competition for nutrients, water and light, such as Wild Oats, which compete for light, water and nutrients in cereal crops. Weeds can be harvested with the crop, reducing quality or spoiling taste and they can also provide food for other pests. Also, weeds can cause the parasitism of crop roots, such as Striga which parasitises maize crops.

What problems are caused by insects, referring to examples?

Insects eat the crop, destroy the crop or they reduce harvests and they also can spoil the appearance of the harvested crop. Aphids, such as greenfly or blackfly, suck the sap and reduce the growth of many crops e.g. cotton, sugarcane, fruit, cereals. Locusts eat almost any plants and termites attack tree crops such as fruit trees. Insects also can act as vectors and spread pathogens e.g. aphids, which carry many pathogenic diseases e.g. potato blight

What problems do fungi cause, giving two examples?

Fungi cause the growing plants or harvested crop to rot e.g. Leaf smut (of rice), Leaf blight (of sugarcane)

What problems do bacteria cause - refer to two examples?

Bacteria reduce harvests by causing disease e.g. bacterial wilt (of potatoes), bacterial leaf blight (of wheat)

How do molluscs cause problems?

Molluscs such as snails and slugs eat the crop, reduce harvests, or spoil appearance

How do Nematode worms cause problems?

They damage roots and reduce water and nutrient uptake and they increase the risks of fungal and bacterial disease. They include Potato cyst nematode and Soybean cyst nematode.

How can vertebrates cause problems?

They eat the growing or harvested crop. Examples include mammals such as mice, rats, deer, rabbits as well as Birds, such as sparrows and quelea.

What is the difference between endemic and epidemic pests?

Endemic pests are pests that are always present, usually in small or moderate numbers while epidemic pests are not normally present but there might be ‘outbreaks’ where they rapidly become a major problem

What is the difference between indigenous and introduced pests?

Indigenous pests are native to the area where they are found while introduced pests are pests that have been introduced from other areas. Introduced pests are often more of a problem as they may not have any predators in the new areas.

What is cultural pest control?

It involves non-pesticide methods where crops or livestock are cultivated in a way that reduces the risk of damage, often by using natural ecosystem services

Describe and explain how crop rotation can help to manage agricultural pests?

Crop rotation involves alternating or changing the crops grown, usually on a four or five year cycle. Pests are often relatively specific to certain crop species and so alternating crops prevents pests establishing colonies and populations - the pests remaining at the end of one year will have died off before that crop is grown again. It also means that if pests lay eggs in the soil these will die before the crop is cultivated again, interrupting the pest reproduction cycle.

How can the use of companion crops manage agricultural pests?

• Companions crops are crops that, when grown together, are more productive. This is due to the crops sharing a symbiotic relationship. For example, some crops can act as a barrier to pest damage.

• An example of companion crops with inter-species relationships are legumes, which can be planted or intercropped among other crops to increase nitrate availability in the soil as legumes increase nitrogen fixation in the soil, enhancing crop growth.

Give some other examples of companion crops

• Barrier crops e.g. the smell of onions can mask the smell of carrots and reduce damage caused by carrot root flies

• Pest attraction to protect the other plants e.g. nasturtiums attract blackfly pests that could damage bean crops

• Support of pollinators - flowering plants that support bees which are important in pollinating fruit crops

Referring to an example, how can providing habitats for natural pest predators manage agricultural pests?

Populations of natural pest predators can be increased through providing suitable habitats. This allows natural predator-prey population dynamics to establish and helps to maintain pest populations at a balanced level. An example of this is that beetle banks and hedgerows can be used to provide habitats and hibernation sites for pest predators such as black ground beetles and ladybirds which eat aphids like greenfly and blackfly.

How can biological control be used to manage agricultural pests and why should they be selected carefully?

Biological control involves introducing natural predator or pathogen species to help regulate pest populations. Biological control species have to be selected carefully - they should be specialist feeders that will eat the pest species and if they have a wider diet then they may eat beneficial and other non-target species.

What is a successful example of biological control?

The Prickly Pear cactus was introduced from South America to Australia where it had no predators and spread rapidly and expansively over farmland. The introduction of the Cactoblastis moth to Australia to manage populations of the Prickly Pear Cactus was successful and both the Cactus and the moth are now rare.

What is an unsuccessful of biological control?

Cane Toads - they were introduced to Australia to control beetle pests of sugar cane but they have eaten a wide range of other species. They are toxic so they have few predators and have colonised a large area.

How do sterile male techniques work to manage agricultural pests?

For many insect species, the females only mate once and then they store sperm for all future egg laying - If the female mates with a sterile male then she will never produce any offspring. Sterile male techniques involve producing many sterile males by exposing them to gamma radiation. These males are then released and if enough males are released then the number of successful matings will reduce to below replacement rate, so the population would decline.

What are some examples of pests that have been controlled by sterile male techniques?

• Screw Worm Fly, which was a parasite of livestock in South America

• Mediterranean fruit fly

• Sweet Potato Weevil in Japan

What factors may effect the success of this technique?

Sterile male techniques are only successful if the sterilised males behave normally and succeed in finding mates. In some projects such as those with mosquitoes, the process of sterilisation using radiation alters their behaviour so that females reject them and still mate with wild fertile males.

How do pheromone traps work to manage agricultural pests?

Pheromones are attractive mating scents and pheromone traps release an artificial scent that attracts pests. These can be used in two ways, either to show that a pest is present so that pesticides can be used to protect the crop; to kill all the pest individuals (or those of one gender) to manage the pest population. They can be used for a wide range of species such as moths, weevils and flies, and is often most successful in enclosed environments like greenhouses.

How can the use of selective breeding maximise agricultural species’ genetic resistance to disease in order to manage agricultural pests?

Selective breeding for disease resistance involves the identification of useful genes which provide characteristics that act as protection against pest species and then using individuals with these genes for breeding in agriculture.

How can crop wild relative species be used to increase genetic diversity in commercial agriculture?

Because commercially cultivated crops have little genetic diversity, to search for new characteristics for selective breeding, Crop Wild Relative Species must be used which can be bred with wild varieties to increase genetic diversity in commercial agriculture.

Give an example of where selective breeding for disease resistance has been used?

Boran cattle in East Africa are more resistant to the protozoan disease East Coast Fever than most other breeds

How does the toxicity of chemical pesticides influence their effectiveness and environmental impacts?

High toxicity pesticides act by inhibiting enzyme action and these are highly effective in killing targeted species, meaning they require the use of smaller volumes. The high insect toxicity of organochlorines led to the deaths of many non-target species such as bees and butterflies and the neurotoxic effect of neonicotinoids has been linked to declines in bee populations. The high mammalian toxicity of organophosphates endangered human health.

How does the specificity of chemical pesticides influence their effectiveness and environmental impacts?

Some pesticides can be more specific than others and these more specific pesticides affect only target species, reducing wider food chain and ecological impacts. For example, pyrethroids are more specific than organochlorines.

How can the persistence of pesticides influence their effectiveness and environmental impacts?

Persistent pesticides are chemically more stable and they degrade more slowly, which reduces the frequency of re-application but can increase the likelihood of a pesticide dispersing more widely in the environment and may extend the time period that it may harm non-target species. Organochlorines such as DDT are highly persistent and they remain in the environment for a long period of time and this often means that they are highly effective in reducing pest levels but also that any environmental impacts are long-term and more severe.

How does a pesticide’s solubility in water influence its effectiveness and environmental impact?

Pesticides that are water soluble are more likely to be washed off a crop, requiring re-application. They may also be more susceptible to leaching into water bodies and may disperse widely to affect non-target areas/species. They might also be diluted if they enter large water bodies, which may reduce their severity.

How does a pesticide’s solubility in lipids influence its effectiveness and environmental impact?

Liposoluble pesticides may be absorbed into crops and this can lead to human ingestion - can enter the human food chain. If a pesticide is liposoluble, such as the organochlorine DDT, it may bioaccumulate and biomagnify up a food chain.

What are the two main modes of action by which chemical pesticides kill pests?

• Contact Action

• Systemic Action

How does the mode of action of a pesticide influence its effectiveness and environmental impact? Contact pesticides.

Contact herbicides kill plants by damaging the tissues that they come in contact with and contact insecticides kill insects that are sprayed directly or come in contact with the pesticide that was sprayed onto the crop that is being protected. The pesticide only protects crop surfaces that have been sprayed, requiring extensive application and the underside of crops’ leaves may not be protected - they can also wash off relatively easily with rain.

How does the mode of action of a pesticide influence its effectiveness and environmental impact? Systemic pesticides.

Systemic pesticides, such as pyrethroids, are absorbed by the crop and translocated throughout the plant. This protects all of the plant and will also protect new growth. The pesticide cannot be washed off by rain but can be retained in the harvested crop and be eaten by humans.

How do hormone pesticides work to manage pests?

Hormone pesticides kill pests through biochemical action, increasing or starting natural processes in a way that is harmful to the pests. Some hormones cause insects to prematurely metamorphose into adults before they are large enough to function, leading to their death. Hormone pesticides have low persistence and are more specific than other types of pesticides.

What are antibiotics and why might they be used?

Antibiotics can be used to protect agricultural yield. They are chemicals that kill microbes such as pathogenic bacteria. They are used in livestock farming to treat infections, to prevent infection through regular dosing, and to promote growth by increasing Gross Growth Efficiency by reducing energy needed for protection against infections and increasing energy used for growth, increasing productivity and farm income.

What are the issues associated with the use of antibiotics in agriculture?

There is growing concern about the over usage of antibiotics in agriculture and growing antibiotic resistance as a result — the use of antibiotics in agriculture is higher than that in human healthcare. Exposure to lower doses of an antibiotic may only kill the most sensitive individuals so the surviving population will be less easily controlled by the antibiotic. The large scale use of antibiotics as growth promoters increases the risks of producing antibiotic resistant bacteria.

What is the agricultural industry trying to encourage to combat this issue?

Only using antibiotics to treat infections rather than as an unnecessary growth promoter.

What is integrated pest management?

Using a holistic combination of measures - each method of pest control has its own advantages and disadvantages and so using a variety of methods balances this.

What are the principles of integrated control?

• Use of cultural techniques which make growth environment less suitable for pests

• Use of cultural techniques that prevent build-up of a pest population

• Cultivating species and varieties that are less vulnerable to pests

• Use of other appropriate non-pesticide techniques

• Use of pesticides when essential with carefully timed applications of specific, non-persistent pesticides e.g. during the breeding cycle of a pest - would have a higher impact